1. Field of the Invention
The present invention relates to electronically driven displays that can translate electrical signals into changeable static images or dynamic video images, and includes multicolor and full color displays. Such displays, or display screens, can comprise a pixellated screen formed by a multitude of individual, selectable state, light-modulating picture elements that can be controlled to provide text or graphic images. More particularly, the invention relates to electrostatic displays which employ capacitive pixels having light-modulating, movable electrodes that can adopt a number of positions, at least one of which is a position extending across the path of a light beam traveling through the pixel. By selective actuation of the movable electrode to interrupt the light beam to a greater or lesser extent, and to change the appearance of individual pixels, groups of such capacitive pixels in the display can be composed into meaningful images.
2. Description of Related Art Including Information Disclosed under 37 CFR 1.97 and 37 CFR 1.98
The above-mentioned parent patents describe, inter alia, a number of capacitively driven, or electrostatic, pixellated video display inventions including, as disclosed in U.S. Pat. No. 5,638,084, an indoor-outdoor multicolor display viewable by transmitted or reflected light. Each pixel of the display employs a movable electrode which, in preferred embodiments takes the form of a miniature metallized plastic coil or spiral while in a relaxed condition. Application of an electrical pulse between the coil and a fixed electrode located on the other side of a dielectric layer from the coiled movable electrode, (termed a xe2x80x9cspiral rolloutxe2x80x9d herein), causes the coil to unfurl across the dielectric layer to modulate light rays striking the pixel, e.g. to block or reflect, them. In effect, the spiral rollout acts as a shutter for the pixel.
Such electrostatic, pixellated displays have advantages of low power consumption, low heat output, and low cost and in some embodiments, of being able to display brilliant reflective images that are viewable outdoors in daylight. Furthermore, preferred embodiments of such electrostatically driven displays are susceptible to mass production from suitably treated low cost polymer film materials. U.S. Pat. No. 5,638,084 discloses a full color video display viewable indoors or outdoors, wherein a mosaicked color screen is aligned behind a black screen comprised by an array of electrostatic shutters each of which registers with a colored or white mosaic element in the color screen.
Suitable drive circuitry for such displays is known, for example, from U.S. Pat. No. 4,336,536 to Kalt and Babcock (xe2x80x9cKalt and Babcockxe2x80x9d herein) which discloses drive circuitry for a pixel display panel which permits selective pixel actuation in rapidly changing, desired groups and patterns of spiral rollouts. Disclosed is a half select row and column drive system, operating in response to timing information extracted by a sync circuit, video information from an incoming signal is supplied to a shift register for loading into columns coupled to the fixed electrodes of the pixels, while row synchronization of the movable electrodes is maintained by a ring counter operating in combination with a plurality of gates, one for each row. The outputs of the column-driving shift register and the row-synchronizing gates are applied directly to the pixels without interposition of further circuit elements.
Kalt and Babcock provides an effective drive system for electrostatic displays, especially displays employing relatively large pixels. One drawback of the system relates to high-resolution or small-pixel displays, e.g. computer or television monitors, where the numbers and density of pixels to be addressed raise potential difficulties in multiplexing the pixel array with an adequate refresh rate, and of possible cross-talk between pixels in adjacent rows or columns leading to unintended actuation of one or more pixels.
The cycle times of electrostatic pixels employing spiral rollout electrodes, of a size of interest for modern video displays, for a cycle including application and removal of an activation voltage, and mechanical rollout and retraction of a coiled electrode, are typically measured in milliseconds, while desired refresh rates are currently at least 30 Hz and for some applications 60 Hz or higher. Since even an EGA screen resolution of 640xc3x97480 pixels contains over 300,000 pixels, it is not practical to allocate a unique time slice to cycle each pixel individually and still achieve the desired refresh rate. The problem is compounded for higher resolutions such as VGA, super VGA and HDTV and for higher refresh rates.
In such higher resolution displays, where row and column conductors are close together, there is a risk that one or more pixel shutters will undesirably respond to a switching signal intended for an adjacent or nearby pixel. It would be desirable to provide suitable drive circuitry which were resistant to cross-talk, even when the display has very small pixels, e.g 0.01 inches (about 250 microns) or less.
The electrostatic displays described above are suitable for a variety of applications, for example, desktop and notebook computer screens, television receivers, conference room or assembly hall presentation screens, instrumentation displays, sports stadium displays (including xe2x80x9cscoreboardsxe2x80x9d with video presentations) and outdoor signage, e.g. highway condition informational signs, as well as smaller personal informational display devices or computer xe2x80x9cappliancesxe2x80x9d such as personal digital assistants, game-playing devices, Internet-enabled cellular phones and so on. Such devices, as they have been known prior to the present invention, because of the limitations of their underlying pixel technology, employ more or less planar or, in the case of cathode ray tubes, mildly curved, fixed form display screens mounted in protective structural frames. The curvature in cathode ray tube displays is usually considered undesirable, and being convex to the viewer, is visually inappropriate, but is more economical and practical for cathode ray technology than are flat-screen displays. Thus, conventional, commercially available displays provide physical restraints which limit the possible range of new devices that may be developed as display-utilizing technologies evolve. For example, a pocket-sized computer device with a form factor of say 8 inches by 4 inches (approximately 20 cm by 10 cm), yet which has a 9-inch (approximately 22.5 cm) diagonal screen is not believed possible with such commercially available displays. To the best of applicant""s knowledge and belief, using conventional technology, the display area of such a device, even ignoring customary margins to the display area, can be no more than the form factor itself, namely, 32 in2 (about 80 cm2). Therefore, it would be desirable to provide a display which can be embodied in a device providing a display area in use which is larger than the form factor of the device in storage, the term xe2x80x9cform factorxe2x80x9d being used to reference the largest of the various possible two-dimensional projected profiles of the device.
The invention solves a problem. It solves the problem of providing an electrostatic display with drive circuitry which can drive high resolution displays with desirable refresh rates and which furthermore is resistant to cross-talk, especially in small pixel embodiments. This problem is solved either by providing self-shielding pixels each having its own Faraday cage, or by employing a unique traveling pulse to address the display raster and apply charge to every pixel to be activated on a given refresh cycle. These innovations are quite compatible, one with the other and may be combined in a display structure, if desired. Also, the invention solves the problem of providing a video display with a large display area relative to its form factor.
Thus, in one aspect of the invention, as applied to a display having a an array or raster of pixels each comprising an electrostatically movable, light-modulating electrode, for example, a spiral rollout, each individual pixel comprises a Faraday cage extending around the zone occupied by the pixel""s movable electrode to shield the movable electrode and reduce cross-talk or cross-coupling between adjacent pixels, which may adversely affect the quality of the displayed image.
Thus each pixel may be self-shielded with its own Faraday cage and, in a row and column display, wherein each pixel has an electrode on either side of a transparent dielectric, can be activated with a front lateral potential and a back vertical potential applied across the dielectric.
This aspect of the invention solves prior problems of lateral charge pollution which have arisen in conventional displays wherein pixels are not shielded. The solution of providing the pixels with individual Faraday cages, rings or margins, is uniquely applicable to electrostatically driven displays.
In one embodiment, such a Faraday cage can created by providing the or each pixel, or each alternate pixel, with a fixed electrode which is larger than the extended movable electrode and which has a marginal zone around its periphery extending laterally beyond the area of the movable electrode in its extended or shuttering position, for example, a fixed electrode opposed to a rectangular spiral rollout, in a row and column display, can have horizontal extensions running between the rows and vertical extensions running between the columns, so that all four sides of the rectangular rollout are shielded. Other configurations of rollout can be shielded by correspondingly shaped and dimensioned fixed electrodes providing extended marginal zones along one or more sides or extensions of the rollout""s excursion area, as will be apparent to those skilled in the art.
To solve the problem of providing an electrostatic display with drive circuitry which can drive high resolution displays with desirable refresh rates, the invention provides in one aspect, an electrostatic display comprising:
a) an array of pixels each including a capacitively drivable electrode shutter movable between an open, light-transmitting position and a closed, light-blocking position and being mechanically retractable to a starting position; and
b) drive circuitry for the pixel array capable of applying control signals to selected pixels to move selected individual ones of the electrode shutters between their respective open and closed positions
wherein the drive circuitry is operable to charge each selected pixel in a relatively short charging interval and to discharge each selected pixel in a relatively longer discharging interval. Preferably, the ratio of the discharging interval to the charging interval is at least 5:1, more preferably at least 50:1 and still more preferably, at least 500:1. By employing a strongly asymmetrical charge:discharge cycle, the invention permits large numbers of pixels to be charged while others are discharging, enabling the complete array to be charged, or addressed for charging if so selected by suitable software drivers, in a small enough time slice, e.g. in {fraction (1/30)}, {fraction (1/60)} second or less, to provide a desired refresh rate.
In a preferred embodiment of the invention, the drive circuitry effects charging by applying a short charging pulse to one or more pixels to be charged, the charging pulse having a pulse width determining the charging interval. Preferably the charging pulse is applied to a dumping capacitor servicing a group of pixels, for example a row or column of pixels, and the dumping capacitor is connected to charge each and every specified pixel in the pixel group on each refresh cycle. Because the charge interval is much shorter than the discharge interval and because no turn-off pulse is required to retract an extended electrode, the charging pulse need not be applied progressively to every designated pixel in the raster during each refresh cycle, but may jump around the display or be applied to one or more groups or rows of pixels in a spatially non-sequential manner, provided that all pixels designated for activation can be charged in a given refresh cycle.
Preferably, the drive circuitry further includes a number of bleed resistances for draining charge from the pixels and each pixel is connectable with one of said bleed resistances to drain charge therefrom. Furthermore, the value of each bleed resistance can be selected, in relation to the capacitance characteristics of the pixel, to determine the duration of the discharge interval. By also taking into account the mechanical characteristics of the movable electrode, an optical ON duration of the pixel, in which the pixel shutter is closed can be determined by appropriate selection of the value of the pixel""s bleed resistance. Commonly, the pixels will be identical one with another, save for their array location, and each will be connected with, or connectable with, the same value of bleed resistance so as to have substantially the same electro-optical characteristics. However, the invention is not so restricted in its application and offers possibilities of varying the bleed resistance as between one or more pixels or groups of pixels in the array and other pixels in the array to provide different ON durations, for special purposes.
In another aspect the invention provides, to solve the problem of reducing or eliminating cross-talk, an electrostatic video display comprising a raster of capacitive light pixels and having a row dumping capacitor connected to each row of the raster to facilitate charging of the pixels in the row. The dumping capacitor preferably has a capacitance comfortably exceeding the sum of the capacitances of the pixels in the row to which it is connected so that it can rapidly receive a charging pulse and subsequently disseminate the charge to individual pixels in the row over a relatively longer time period.
In a still further aspect, the invention provides a cross-talk resistant electrostatic video display comprising a raster of rows of capacitively driven light-modulating pixels having distinct, logical pixel addresses, and drive circuitry to provide charging pulses to selected pixels according to an intended display image, wherein the drive circuitry provides an image refresh cycle and generates in each cycle, a traveling pulse which scans the raster, encompassing every pixel address once every cycle. In one preferred embodiment, the traveling pulse advances through the raster in a progressive manner traveling sequentially along one row and then to the next adjacent row until the complete raster has been scanned. Preferably, the traveling pulse traverses each row in the same direction, rather than moving sinuously across the raster, and avoids simultaneously addressing a given column address in adjacent rows, so as to reduce the probability of inadvertently activating a pixel not specified for activation in the video software or signal.
Thus, a pixel-charging activation pulse can be applied to all specified ones of a horizontally contiguous pixel group, wherein the pixel group moves progressively from row to row across the raster to visit every pixel address during each refresh cycle, the number of pixels in the group preferably being substantial, e.g. 10 or more, but being substantially less than or, at most, equal to the number of pixels in a row, so that specified row-adjacent pixels receive charging pulses at different times, whereby cross-talk is inhibited.
Effectively, the invention provides, in this aspect, a row-sequential charging system which reduces the risk of cross talk by avoiding simultaneously delivering a charging pulse to pixels with the same column address in adjacent rows, while avoiding the delays that would occur if the pixels were charged one at a time. However, some pixels at one end of one row may be charged at the same time as some pixels at the other end of an adjacent row are being charged, so long as there is no overlap with respect to the columns. Alternatively, the traveling pulse could be applied to the columns. It will be appreciated that for the purposes of the present invention, the rows and columns may be logically interchangeable, subject to the requirements of the video signal which usually scans horizontally. In either configuration, cross talk is avoided by the invention, by maintaining a spatial or temporal separation of pixels receiving a charging pulse so that the act of charging one, or more significantly, several adjacent pixels in one row, (or column) does not trigger a quiescent pixel in an adjacent row (or column respectively).
As an alternative to a row-sequential charging regimen, the pulse could jump around the raster being applied to different groups of pixels in turn according to a random or organized pattern. The ability to charge a pixel very quickly enables complex, randomized, or partially randomized, charging patterns to be employed wherein, for example, an individual pixel, or small group of possibly non-adjacent pixels in a first portion of the raster is pulsed, then left to unfurl while another pixel or group in a second portion of the raster, which may be remote from the first portion of the raster, is pulsed in its turn, and so on until the complete raster is addressed pursuant to the randomized or partially randomized pattern. It will generally be more convenient to repeat the pattern in each refresh cycle, but variations may be made as will be apparent to those skilled in the art. A suitable group size will also be apparent to those skilled in the art and may for example comprise from about 0.001 percent to about 5 percent of the total number of pixels in the display, preferably about 0.05 to about 1 percent. If desired, the group can be an organized group, for example a geometric sub-unit of the complete display raster, for example a rectangle, triangle, hexagon or a complete row or column, and not all groups need have the same number of pixels or geometric character. To use illustrative language, the pulse can be envisaged as hitting a few pixels in one place, delivering a charge then vanishing to hit a few pixels in another place, and then another in a fast-moving, more or less complex, randomized pattern.
Preferably, the circuitry component characteristics are selected so that the complete display area can be refreshed within a desired interval, for example, for a typical desktop computer, or domestic television display, at least every one-thirtieth ({fraction (1/30)}) of a second, although a refresh rate of 60 or even 100 or more hertz is more preferable.
Typically, each pixel comprises a movable electrode and a fixed electrode and in this case a further preferred feature of the inventions is to connect the movable electrode to the drive circuitry through a bleed resistor or other means or device to delay bleed-off of charge from the pixel. If desired, a radio frequency choke can be provided for each pixel row to inhibit drive pulses from traveling in an unintended row.
Pursuant to the invention, it has been found that the mechanical response of a spiral rollout, a preferred configuration of movable electrode in an electrostatically driven display, exhibits in its mechanical response, a hysteresis lag behind applied electrical driving voltages. Thus, the rollout does not begin to uncoil from its coiled-up, relaxed state, until an activation threshold voltage is reached, whereupon the mechanical response continues after the charge is removed. The charge necessary to effect complete extension of the rollout (to close the shutter and render the pixel reflective) can be applied in a very short interval, perhaps a few microseconds, while the mechanical extension of the electrode induced by application of the charge may have a much longer duration, perhaps a number of milliseconds. The present invention exploits such electromechanical hysteresis characteristics in novel ways to provide beneficial new constructions of display that are not possible with conventional non-electrostatic displays, such as cathode ray, liquid crystal, active matrix and so on, which do not exhibit such characteristics. A further advantage of employing as movable electrodes mechanically biased spiral rollouts is that no negative pulse is required to retract the shutter, which enables simplified drive circuitry to be employed. When the charge decays to a release point below threshold, the extended electrode simply rolls up under the mechanical stresses induced in manufacture, which bias it to a coiled, retracted configuration.
Thus, unlike other types of display, in an electrostatic display employing movable pixels, a biasing voltage can be used. For example, referencing a row-and-column display, each row of pixels can be supplied with a biasing direct current voltage to a value below or near a pixel activation threshold to reduce the required drive signal voltage.
The several aspects of the invention are well adapted to be embodied in displays wherein the pixels are organized in an orthogonal array and a half-select drive system is employed. In such displays, the fixed electrodes can be connected together in columns extending transversely of the pixel rows, preferably located behind the dielectric, from the viewer""s perspective, while the movable electrodes are connected together in rows. The timing and level of the applied voltage are selected on a row-by-row basis so as to apply a proportion of the applied voltage, for example half, to columns with an active address and then to apply a desired complementary voltage to the row so that all pixels in the row with an active address are charged. To achieve the desired traveling pulse effect of the invention, it may be preferable first to apply the row voltage, and then to apply the column pulses in sequence as a traveling group moving from one end of the row to the other.
To facilitate the process of charging specified pixels, the drive circuitry preferably further includes a column dumping capacitor for each column of pixels and a drain resistor connected in parallel across the column dumping capacitor to leak charge across the dumping capacitor and drain static build up.
In preferred embodiments, the drive circuitry includes a power source and, for each row and transverse column of pixels, a clock switch to connect the row or column to the power source. If desired, the whole display can be enclosed in a Faraday cage to inhibit stray external electrostatic events from interfering with proper operation of the display.
Larger displays, such as those for a theater, sports stadium or outdoor arena, can be constructed as a large matrix divided into rectangular panels that are separately driven and electronically pasted together to generate a coherent image.
In another aspect, to solve the problem of providing a portable display device having a useful display area which is larger than its form factor (the device""s largest projected profile or footprint) in a closed, or out-of-use configuration of the device, provides a display with a variable configuration viewing or image area incorporated in a flexible, foldable or bendable structure providing the viewing area, whereby the display can adopt a compact out-of-use configuration, and can be opened up to provide an extended display area. A simple example of such a display is a book-like device, e.g. a notebook computer, wherein the display area extends across both leaves of the book to provide an area greater than an individual leaf. The invention also provides a display which is shaped rather than flat, or both flexible and shaped, and is a non-planar, thin-panel video display which may comprise a display raster, or matrix. In a preferred embodiment, such a shaped or flexible display comprises a polymer film laminate of electrically addressable and activatable pixels.
Preferably, the pixels of such a variable form display are electrostatically actuated and the polymer film laminate comprises a first polymer film providing a dielectric layer, a pattern of fixed electrodes contacting said dielectric layer and a second polymer film providing a movable electrode layer wherein the movable electrode layer comprises a corresponding pattern of movable electrodes to the pattern of fixed electrodes, the movable electrodes being movable across a light path through the pixel to modulate light rays traveling on said light path.
While consistently with the teachings of the present invention, display constructions employing electrostatically actuated pixels are preferred for the advantages they bring, those skilled in the art may, with the benefit of the teachings herein, devise other video displays which can be embodied in shapable or flexible materials for example polymer films and such other video displays having a useful shaped or flexible configuration are contemplated as coming within the purview of the present invention.
As referenced above, a preferred configuration of movable electrode for use in the above-described aspects of the invention comprises a spiral rollout pre-stressed to a coiled-up position. As is known from my prior patents and applications, such a spiral rollout can be provided with a metallic coating on a surface facing the dielectric so that application of a suitable charge to the capacitive pixel causes the spirally wound electrode to be attracted to the dielectric and to roll out, closing the shutter. This device has the advantage that no negative pulse is required to retract the shutter to an optically open position. When the charging pulse is released, or decays, the electrode rapidly retracts to its coiled-up position, in a spring-like manner. Additionally, the coiled electrode has a small footprint in its retracted position compared with its extended position so that there is little passive or dead space within a display perimeter and the active pixel areas fill a high percentage of the occupied display area. For example, smaller spiral rollouts may have lengths 10 or more times their retracted diameters, while for larger rollouts the proportion may be 100 or more.
Other configurations of movable electrode suitable for operation in an electrostatic pixel will be apparent to those skilled in the art, or may be developed, for example equilateral triangular rollouts organized in hexagonal arrays and xe2x80x9cflapperxe2x80x9d-type pixels, the latter of which are known and have been utilized in outdoor displays.